This invention generally relates to a superplastic duplex-phase ferrous alloy suitable for superplastic working and to a superplastic hot working method therefor. More particularly, it relates to an inexpensive nitrogen-containing ferrous duplex-phase alloy for superplastic working which exhibits two phases consisting of a ferrite phase and an austenite phase at temperatures near 1000.degree. C., and a superplastic hot working method therefor. This invention also relates to a superplastic hot working method for a duplex-phase stainless steel which exhibits two phases consisting of a ferrite phase and an austenite phase near room temperature and which has Fe, Cr, and Ni as main components.
It is known that duplex-phase ferrous alloys including duplex-phase stainless steels which consist of a ferrite phase (.alpha.) and an austenite phase (.gamma.) generally have excellent strength, toughness, and weldability. For this reason, in recent years, they have come to be used in a wide variety of fields, and the demand therefor has been increasing. However, the presence of these two phases also causes these steels to be difficult to work.
Accordingly, in order to improve the workability of this type of duplex-phase ferrous alloy, in the past, countermeasures have been taken such as reducing the amount of impurities such as sulfur (S) and oxygen (0) which are harmful to hot working. At present, it has become possible to perform hot working of such ferrous alloy in the manufacture of simple shapes such as pipes and plates and forgings having relatively simple shapes. However, the manufacture of parts with complicated shapes such as pipe joints and valves from a duplex-phase ferrous alloy by hot working alone is still extremely difficult, and it is necessary to rely on machining and molding processes which have a poor yield or efficiency.
In recent years, much research has been performed on superplastic working technology as a method of forming such difficult to work materials into complicated shapes. It has been reported that a duplex-phase ferrous alloy, such as duplex-phase stainless steel which contains large quantities of Cr, Mo, and Ni and which is difficult to work by the conventional hot working exhibits remarkable superplasticity [see "Iron and Steel", Japanese version, 70, (1984) pp. 378-385]. The superplastic working method reported therein employs a superplastic phenomenon accompanying the precipitation of the .sigma.-phase in a duplex-phase stainless steel having a composition of Si: &lt;0.48%, Mn: &lt;1.60%, Ni: 5.5-7%, Cr: 21-25%, Mo: 2.7-2.8%, and N: at most 0.15%. As a result of such research, the common idea up to the present time that it is difficult to utilize superplasticity with duplex-phase ferrous alloy has been disproven, and the technology related to its superplastic working is constantly developing. In addition to the previously-described mechanical properties and weldability, this type of duplex-phase stainless steel exhibits excellent corrosion resistance, and products manufactured from such duplex-phase stainless steel by superplastic working are highly suitable, for example, for use in sea water such as for seawater-resisting instruments and parts for drilling oil wells, although the superplastic working has to be carried out at a relatively low strain rate with heating.
However, this type of duplex-phase stainless steel contains relatively large amounts of Cr, Ni, and Mo, making it expensive. Therefore, there is a limit to its uses, and there is a strong desire for the development of an inexpensive material having excellent superplasticity which is a general ferrous alloy and which can be used in products not requiring excellent corrosion resistance.
When performing superplastic working on the above-described duplex-phase stainless steel which contains relatively large amounts of Cr, Ni, and Mo, it is generally necessary that the strain rate during working be low in order to attain superplasticity. Therefore, not only does superplastic working require a relatively long time, but it is necessary to perform working while heating in order to prevent a decrease in temperature during working, both of which decrease manufacturing efficiency and increase costs.